The present invention relates generally to thermal barrier coatings and coating systems, and relates more particularly to coatings and coating systems having improved durability.
Gas turbine engines are well developed mechanisms for converting chemical potential energy, in the form of fuel, to thermal energy and then to mechanical energy for use in propelling aircraft, generating electric power, pumping fluids, etc. The metallic materials used in gas turbine engines are currently very near the upper limits of their thermal stability. In the hottest portion of modern gas turbine engines, metallic materials are used at gas temperatures above their melting points. They survive because they are air cooled. But providing air cooling reduces engine efficiency. Accordingly, there has been extensive development of thermal barrier coatings for use with cooled gas turbine aircraft hardware. By using a thermal barrier coating, the amount of cooling air required can be substantially reduced, thus providing a corresponding increase in efficiency.
Generally speaking, metallic materials have coefficients of thermal expansion which exceed those of ceramic materials. Consequently, one of the problems that must be addressed in the development of successful thermal barrier coatings is to match the coefficient of thermal expansion of the ceramic material to the metallic substrate so that upon heating, when the substrate expands, the ceramic coating material does not crack. Zirconia has a high coefficient of thermal expansion and this is a primary reason for the success of zirconia as a thermal barrier material on metallic substrates.
Thermal barrier coatings are deposited by several techniques including thermal spraying (for example plasma, flame and HVOF), sputtering and vapor deposition such as electron beam physical vapor deposition (EBPVD). Of these techniques, electron beam physical vapor deposition is currently a preferred technique for demanding applications because it produces a unique coating structure. Electron beam physical vapor deposited ceramic materials, when applied according to certain parameters, have a columnar grain microstructure consisting of small columns separated by gaps which extend into the coating. These gaps allow substantial substrate expansion without coating cracking and/or spalling. See, e.g., commonly owned U.S. Pat. No. 4,321,311. According to U.S. Pat. No. 5,073,433 and commonly-owned U.S. Pat. No. 5,705,231, a similar structure (comprising segmentation cracks) although on a larger scale, can be obtained by plasma spray techniques.
In order to protect the substrate, the thermal barrier coating must continue to adhere to the substrate, or an intermediate bond coat, during the service life of the component. However, repeated thermal cycling of coated components results in repeated thermal cycling of the ceramic and metallic materials, and thermally induced stresses and strains. Failure of the system may occur at the metal/ceramic interface, between the bond coat and ceramic, or the substrate and ceramic where no bond coat is used. Such failure usually results in spallation of the coating, exposure of the underlying substrate, followed by failure of at least a portion of the underlying component.
One attempt to overcome the spallation problem is set forth in U.S. Pat. No. 5,419,971. The ""971 patent asserts that grooves provided, e.g., in the bond coat, arrest cracks forming in the ceramic in the area of the ceramic/bond coat interface. The grooves are preferably provided in the bond coat using a pulsed laser, or other suitable process for selectively removing small amounts of the metallic bond coat material. The laser purportedly ablates material with xe2x80x9cvirtually no effect on the underlying material that is not removedxe2x80x9d. Other suitable methods include mechanical micro-machining and engraving processes. A conventional photoengraving process is described at col. 6, lines 24-60. An acid resistant coating is applied to substantially all of the substrate, except for a small portion, and the uncovered portion is etched with an acid to form the grooves. The acid-resistant covering, which prevents the acid from contacting most of the substrate, is then removed from the substrate.
The ""971 patent specifically teaches that conventional chemical etching, in which the entire surface is exposed to etchant, is unsuitable in providing grooves or other features in the metallic material. According to the ""971 patent, conventional chemical etching results in contamination and/or cracking the underlying material. See. e.g., col. 3, line 58-col. 4, line 8.
Testing of the subject matter discussed in the ""971 patent indicates that a layer of highly disturbed material, e.g., re-cast material, exists along the outer surface of the grooves, and that the geometry of grooves are typically highly irregular. In the case of material removal by laser, the remaining re-cast material tends to be chemically altered by selective evaporation of chromium and also some aluminum and yttrium. Moreover, the re-cast material also tends to include a very fine grain structure, which exhibits poor high temperature strength relative, for example, to single crystal substrate materials and even to conventionally cast substrate materials. The highly disturbed material and the geometric irregularity can result in TBC durabilities that are significantly below TBCs that do not have grooves or similar features. While the layer of disturbed material may be thicker or thinner, depending upon the manner in which material is removed to provide the grooves or other features, the layer of disturbed material exists and detrimentally affects the performance of a TBC system.
Although the present coating was developed for application in gas turbine engines, the invention clearly has utility in other applications where high temperatures are encountered, such as furnaces and internal combustion engines.
It is an object of the present invention to enable an improved method for providing grooves or other features in bond coats/substrates of TBC systems such that the TBC systems have improved durability.
It is another object to provide TBC systems that have grooves or other features in bond coats/substrates with improved durability.
Additional objects will become apparent to those skilled in the art based upon the following drawings and description.
According to one aspect of the invention, a method is disclosed for providing an article with a thermal barrier coating system having improved durability.
The method includes the steps of providing a substrate having an exposed surface; removing preselected portions of substrate material from the exposed surface of the substrate to provide predetermined three dimensional features on the substrate, with the step of removing preselected portions typically resulting in at least some adjacent areas of material which are disturbed; chemically treating the entire exposed surface of the substrate including the features to remove any disturbed material; and applying a layer of thermally insulating ceramic material to the substrate.
An article made in accordance with the method is also disclosed.
The present invention enables the production of TBC systems having significantly improved durability. Testing indicates TBC systems prepared according to the present invention have a 5xc3x97 design life improvement over corresponding TBC systems with untreated grooves. While the present invention requires some additional work in the production of the TBC systems, the additional steps do not add significantly to the costs of production, and any additional cost is substantially outweighed by the improved durability.